Phosphites represent a diverse class of organic phosphorus compounds that are useful as ligands for homogeneous catalysis and as components of plasticizers, flame retardants, UV stabilizers and antioxidants. Phosphites can be further classified as organomonophosphites and organopolyphosphites. Organopolyphosphites are particularly useful for certain homogeneous catalysis; for example, U.S. Pat. No. 4,769,498 generally relates to synthesis of organopolyphosphites and use thereof as ligands in hydroformylation processes.
Phosphoromonochloridites are intermediates for synthesizing organopolyphosphites; see, for example, U.S. Pat. Nos. 6,031,120, 5,663,369, and 4,769,498. A phosphoromono-chloridite is typically synthesized in a condensation reaction by contacting phosphorus trichloride (PCl3) with one molar equivalent of a di-alcohol or two molar equivalents of a mono-alcohol under reaction conditions highly dependent on the reactivity of the starting alcohol and the resulting phosphoromonochloridite. For each molecule of a phosphoromonochloridite produced, the condensation reaction produces two molecules of hydrogen chloride (HCl). In order for the condensation reaction to achieve high, for example, greater than 90 percent, conversion of the alcohol, HCl needs be removed from the reaction solution.
One approach for HCl removal from the condensation reaction is to neutralize HCl using a nitrogen base, in an amount stoichiometric to or in excess to the theoretical amount of HCl to be produced. See, for example, U.S. Pat. Nos. 5,235,113; 6,031,120, and 7,196,230, U.S patent application publication 2007/0112219 A1, and Journal of Molecular Catalysis A: Chemical 164 (2000) 125-130. When a nitrogen base is used, however, the resulting nitrogen base-HCl salt must be removed from the reaction mixture by a filtration procedure, which generates chloride and nitrogen-containing wastes that, in turn, increase cost.
Another approach for HCl removal from the PCl3-alcohol condensation reaction involves heating a mixture of the alcohol and a large excess amount of the PCl3 at a temperature sufficiently high to reflux PCl3 (boiling point (bp): 74-78° C.), which drives off the HCl. In this approach, the nitrogen base is not needed or used. For example, U.S. Pat. No. 4,769,498 discloses a procedure for producing 1,1′-biphenyl-2,2′-diyl phosphoromonochloridite by refluxing a mixture of 2,2′-biphenol with 3.7 molar equivalents (2.7 equivalents in excess) of PCl3. The phosphoromonochloridite product is disclosed to be isolated in 72 mole percent yield, based on moles of 2,2′-biphenol employed, by distillation under reduced pressure. Another procedure, as referenced in Korostyler et al., Tetrahedron: Asymmetry, 14 (2003) 1905-1909, and Cramer et al., Organometallics, Vol. 25, No. 9 (2006) 2284-2291, synthesizes 4-chlorodinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine by heating a mixture of 1,1′-bi-2-naphthol and 11.5 molar equivalents of PCl3 at 75-80° C. One undesirable feature of the aforementioned approach involves the need to remove and handle a large excess amount of PCl3, which reacts exothermically with moisture and typically involves additional safety considerations. It would be desirable to reduce the excess amount of PCl3 to be used in the process.
When the PCl3-alcohol condensation reaction uses a solid diol, yet another approach for HCl removal involves: a) dissolving the solid diol either in an aprotic polar organic solvent, preferably tetrahydrofuran (THF), or in a solvent mixture comprising an aprotic polar organic solvent, to produce a feed solution; and b) adding the feed solution slowly into a refluxing solution of PCl3 dissolved in a hydrocarbon solvent, such as toluene. The refluxing is required to drive off the HCl as a gas from the reaction solution. The aprotic polar organic solvent, such as THF, is generally required to obtain a feed solution containing greater than about 20 weight percent of the diol at ambient temperature, based on the weight of the feed solution, particularly if the diol has an unacceptable solubility in the hydrocarbon solvent. This process has been used commercially and is the subject of U.S. Provisional Patent Application 61/040,304 , filed Mar. 28, 2008, for “ISOTHERMAL PROCESS FOR PHOSPHOROMONOCHLORIDITE SYNTHESIS,” filed in the name of Union Carbide Chemicals and Plastics Technology LLC.
With reference to the aforementioned commercial process, hydrogen chloride is known to react with the preferred aprotic polar organic solvent, namely, tetrahydrofuran, to produce 4-chlorobutanol; see, for example, Barry et al., Journal of Organic Chemistry (1981), 46(16), 3361-4. Hydrogen chloride reacts with tetrahydrofuran more slowly at lower temperatures, if other conditions remain the same; however, operating at temperatures lower than about 98° C. can lead to accumulation of hydrogen chloride in the reaction solution in the form of THF-HCl complexes, which in turn can lead to an even higher rate of 4-chlorobutanol production. Disadvantageously, both PCl3 and the phosphoro-monochloridite react with 4-chlorobutanol to produce undesirable by-products.
The phosphoromonochloridite is desirably used without further purification in the synthesis of organopolyphosphites. Formation of 4-chlorobutanol, however, during the phosphoromonochloridite condensation reaction not only reduces the yield of phosphoromonochloridite product, preferably, 1,1′-biphenyl-2,2′-diyl phosphoromonochloridite, but also complicates subsequent organopolyphosphite synthesis reactions.
As a further aspect of the aforementioned commercial process, any mixture of PCl3 and THF recovered from the process typically is not reused due to the need to separate PCl3 (bp: 74-78° C.) and THF (bp: 65-67° C.).
In view of the above, a need exists in the art for a more efficient process of producing a phosphoromonochloridite.